This invention relates to a digitizer for a gravity meter having a gravity readout dial and a null meter and, in particular, to a digitizer that monitors the rotation of the readout dial and produces and stores a digitized output signal representing the gravity at a particular location in the form of the average position of the readout dial on the gravity meter.
All gravity measurements necessarily are made in the gravitational field of the earth. Therefore, a knowledge of this field is required so that proper allowance for it can be made in reducing any kind of gravity measurement to a form useful for indicating geologic structure. The earth is not a perfect sphere. To a rather close approximation, its shape is that of a perfect fluid for which a balance is maintained between the gravitational forces tending to make it spherical and the centrifugal forces of rotation tending to flatten it. As a result of this balance the equatorial radius is about 21 kilometers greater than the polar radius. This flattening means that the acceleration of gravity is greater at the poles than at the equator.
For the application to geophysical prospecting, the interest in the gravity formulas lies in their use in making corrections for the normal northward or southward gravity increase. If the earth were a perfect fluid with no variations in density, its surface would correspond to an ideal ellipsoid of revolution, the so-called normal spheroid represented by the gravity formula. This would be a level surface and the direction of gravity everywhere would be perpendicular to this surface. Actually, as pointed out, the earth is not uniform in density and there are departures of the level surface from the normal or reference spheroid. The actual level surface may be considered as that of the oceans and the oceans as extended across land areas by imaginary deep canals. The level surface so defined is the "geoid." An ordinary level indicates a surface parallel to the geoid. A plumb line gives a direction normal to the geoid. The actual level surface is deformed or warped by the irregularities in density within the earth and the topographic irregularities of its surface. These deviations or deflections of the vertical can be determined by certain geodetic and astronomic measurements and also by surface integration of the gravity field around the point of calculation.
One of the instruments utilized for making gravity readings is a gravity meter such as those made by LaCoste & Rhomberg. These gravity meters, if utilized properly, indicate the gravity at a particular station or point.
To utilize these gravity meters, a very particular and precise procedure must be followed. First, the power must be applied to the meter and the gravity meter mechanism must be allowed to achieve thermal stability by allowing sufficient time to elapse after application of power to an internal heating element. During the time the instrument is being stabilized thermally, it must be carefully levelled in two axes using bubble levels. After the instrument has achieved thermal stability, a caging mechanism is released allowing the mechanism inside of the gravity meter to become operative. A coarse measurement of the nulling of the meter is obtained by rotating the vernier dial and utilizing an eyepiece and a scale located inside the instrument. The gravity meter vernier dial is then rotated until an error signal on a galvanometer is nulled for a fine measurement. After the fine error signal null is obtained, all of the levels have to be rechecked and again there must be a check for any drift in the error signal. The gravity reading is then written down as a combination of the vernier dial reading and an odometer style mechanical readout. Recording of the time of the reading, the day of the reading, and the location where the reading is obtained is done manually. The time and date are recorded so that gravitational effects of the moon and sun upon the meter at its latitude, longitude and elevation may be calculated and the gravity corrected for these effects.
A number of problems are encountered which cause reading errors in the above procedure to be obtained. First, seismic activity or nearby movement of vehicles can make it difficult to properly null the error signal since the instrument is so sensitive that the components continually move. Second, the gravity meter level adjustment may be disturbed inadvertently by the time all other adjustments are made and a reading is taken due to the earth's rotation and thus the change in lunar position and effect. Third, bubble level indicators are subject to interpretation error. Fourth, mechanical shock encountered in moving the gravity meter from one location to another can induce temporary or permanent reading error possibly accompanied by a reading error drift over a period of time. Fifth, nulling the error signal by means of the built-in indicators is subject to interpretation errors by the human operator, particularly when seismic or other disturbances are present as discussed earlier. Sixth, thermal stability may not be sufficiently complete for a variety of reasons such as failure to allow sufficient heating time since the last cool down which may be a matter of hours, exposure of gravity meter to excessive wind, sun or other thermal sources, and weakening of the batteries which provide power for the heater and the thermal control system within the gravity meter. Seventh, caging the mechanism may induce temporary error and drift in the gravity readings requiring a sufficient time to elapse before the reading is taken. Eighth, the gravity reading taken from the gravity meter may be misread and/or incorrectly logged. Ninth, the time of day and/or date of the reading and the location or position at which the reading is taken may be incorrectly logged.
The gravity meter digitizer of the present invention reduces these errors in several ways. First, the gravity meter reading is determined by electronically counting the precise number of turns of the gravity meter vernier dial (including fractional turns) and thus the dial reading is more accurately measured by the system than when observed by the human operator.
Second, failure of the operator to exactly null the error signal is compensated for by disallowing or prohibiting a gravity reading from being recorded if the error signal is not within predetermined tolerance limits and informing the operator by means of status indicators. Also, any error signal value that remains within tolerance limits will be recorded with the gravity reading itself so that compensation for the residual error may be made during the data analysis phase.
Third, any error signal variation due to seismic or local disturbances is reduced in the microprocessor element by means of a commonly known digital filtering and averaging algorithm utilizing a succession of samples of the error signal. The characteristics of this filtering/averaging algorithm are programmable via a storage/command unit.
Fourth, drift in the error signal due to thermal or mechanical effects as described earlier is detected by the same filtering/averaging algorithm. Recording of the gravity reading may be inhibited and status indicators will inform the operator.
Fifth, electronic level sensors are monitored by the microprocessor element. If the level sensors provide signals indicating that the spatial orientation of the gravity meter is out of predetermined allowable limits, status indicators tell the operator that the recording of the gravity signal is being inhibited. If the spatial error signal is within the predetermined tolerance limits, the residual level error is recorded with the gravity reading thus allowing compensation for its effect during data reduction.
Sixth, thermal conditions are monitored at several points within the gravity meter allowing detection of abnormal thermal gradients or drifts. Status indicators again inform the operator and may inhibit the recording of the gravity signals if the temperature exceeds the limits. If, however, the temperatures are within tolerance, the residual temperature error is recorded simultaneously with the gravity gradient signal again allowing compensation for its effect during data reduction.
Seventh, the gravity meter heater power is monitored by the microprocessor element and the value is recorded with the gravity readings. Out of tolerance values cause status indicators to warn the operator and inhibit the gravity readings from being recorded. If, however, the power limits are within predetermined tolerance limits, the residual power error is recorded with the gravity gradient reading again allowing compensation for its effect during data reduction.
Eighth, acceleration sensors are located on the gravity meter to detect acceleration due to forces other than gravity and are monitored by the microprocessor element and the number and severity of these forces, such as mechanical shocks received by the gravity meter, are stored and reported with the gravity readings. Excessive amounts of shock exceeding predetermined limits may cause the status indicators to warn the operator and may inhibit the recording of the gravity signals. Again, any residual acceleration error that remains within tolerance limits is recorded with the gravity gradient in order that compensation for its effect may be applied during data reduction.
Ninth, the precise time of day and date as taken from the internal clock is stored with each gravity reading recorded. Tenth, all of the above-described data making up a gravity reading is recorded electronically in the microprocessor and may be accessed for storage in an external unit as necessary. The stored signals may then be transferred to a computer for the necessary calculations.
All time, date, filter/averaging characteristics, tolerance limits (levels, temperature, power supply, error signal) and calibration functions are programmable by use of an external storage/command unit. A system of access codes or pass words associated with each of the above functions protects all parameters from unauthorized or accidental modification. Recording inhibition due to any parameter exceeding its tolerance limit may be overridden by special command. Recorded data is tagged appropriately to identify the particular parameter out of limits and its value.